This invention is in the field of projection display systems, and is more specifically directed to the recovery of reflected light in a digital micromirror device display system.
As is evident from a visit to a modern electronics store, the number of flat-panel (i.e., non-CRT) televisions has vastly increased in recent years, while the purchase price for such sets continues to fall. This tremendous competition is due in large part to the competing technologies for the display of high-definition television content. As known in the art, three major current display technologies for flat-panel televisions include liquid-crystal display (LCD), plasma display, and digital micromirror (DMD) based displays. The micromirror-based displays, and some LCD displays, are projection displays, in that a light source illuminates a spatial light modulator formed by the micromirror or LCD panel, with the modulated light then optically projected to a display screen. Plasma displays, on the other hand, are not projection displays; rather, each pixel at the display screen includes red, green, and blue phosphors that are individually excitable by way of argon, neon, and xenon gases, producing the image. Some LCD televisions involve “direct-view” displays, in which the liquid crystal elements at the display screen are directly energized to produce the image.
In modern micromirror-based projection displays, such as DLP® projection displays now popular in the marketplace using technology and devices developed by and available from Texas Instruments Incorporated, a digital micromirror device spatially modulates light from a light source according to the content to be displayed. An optical “engine”, which includes lens and mirror elements, projects the modulated light onto the display screen. As known in the industry, micromirror-based projection displays are advantageous from the standpoint of brightness, clarity, and color reproduction, as compared with other flat-panel televisions and displays. In addition, micromirror spatial light modulators enable higher-speed modulation of light than many LCD systems, and micromirror-based systems have been observed to be extremely reliable over time.
Modern micromirror-based displays project color images by sequentially illuminating the spatial light modulator with light of three or more primary (e.g., red, green, blue) colors within each frame period, so that the spatial light modulator sequentially projects images of these primary colors within that frame period. Assuming that the frame period is sufficiently short, the human eye will integrate the sequential primary color images into a single full-color-image. The illuminating primary color light is conventionally generated by a white light source illuminating a rotating “color wheel”, or by three or more monochromatic light sources (e.g., lasers) operating sequentially or simultaneously within the frame period. In either case, however, the light illuminating the spatial light modulator is a fraction of the total light generated within the display system. As a result, single-modulator sequential color display systems project images that have low color content relative to the power available in the system. This inefficiency is present not only in single-modulator micromirror-based systems, but also in sequential-color display systems using other types of spatial light modulators (e.g., LCD). This type of inefficiency can be avoided by providing a spatial light modulator dedicated to each primary color, so that all colors are simultaneously displayed throughout the entire frame period, at 100% duty cycle. However, the multiple-modulator display system is of course much more expensive, because at least three spatial light modulator chips are required, and because of the complex construction necessary to attain precise alignment and superposition of simultaneously projected image color components.
To reduce the effects of this inefficiency in light usage in single-modulator display systems, conventional micromirror-based display systems often include recycling techniques in the illumination path (i.e., prior to the spatial light modulator). These techniques recapture the light of the primary colors (e.g., red, green, or blue) other than the one currently illuminating the spatial light modulator. Examples of conventional light recycling approaches include rod integrators, such as described for example in U.S. Pat. No. 7,052,150 and U.S. Pat. No. 7,184,213, both commonly assigned herewith and incorporated herein by this reference. Conventional sequential color recapture (SCR) techniques are described in U.S. Pat. No. 6,771,325 and U.S. Pat. No. 7,118,226, both commonly assigned herewith and incorporated herein by this reference. Commonly assigned U.S. Pat. No. 6,642,969, incorporated herein by this reference, describes a spiral color wheel for improving the efficiency of light utilization in the illumination system, in combination with dichroic filters to reflect out-of-band light in a sequential color recycling display system. SCR thus refers to the recycling of light before it leaves the illumination module; in other words, SCR techniques recycle light that has not yet illuminated the spatial light modulator.
Another source of inefficiency in light usage is present in micromirror-based displays, as a result of the DMD spatial light modulator reflecting “off” pixel light away from the display screen. For example, as shown in the above-incorporated U.S. Pat. No. 7,184,213, the light for “off” pixels reflected from the digital micromirror is directed to a “light dump”, which is a light-absorptive element that keeps this unused light from scattering within the system, which would reduce contrast in the displayed image. Of course, light directed to the light dump is of no further use in the system. This invention is directed to the recovery of this “off” pixel light, as will be apparent from the description provided herein.
By way of further background, contrast in the image displayed by micromirror-based projection systems is degraded by interference between the light of “on” pixels, reflected from the spatial-light modulator at one angle, and the light of “off” pixels that are reflected at another angle. The likelihood or extent of this interference of course depends on the angle to which the micromirrors in the spatial light modulators are deflected in those states. FIG. 1 schematically illustrates the paths of light illuminating and reflected from micromirror M in a conventional micromirror-based projection system. In this schematic illustration, micromirror M is capable of deflecting from a flat state (i.e., undeflected) to either an “on” state or an “off” state, each of such states being at an angle of 10° from the flat state. As known in the art, some modern digital micromirrors now have ±12° angles of deflection. In this display arrangement, light source L illuminates micromirror M, via illumination cone IC, at a nominal angle of −20° relative to its flat state. Of course, the angle of reflection of the illuminating light from micromirror M equals the angle of incidence. As such, with micromirror M is in its flat state, the projection pupil FLAT reflected by micromirror M is at an angle of +20°, because the incident light is at −20° relative to this flat state. For an “on” state pixel, micromirror M is deflected to an angle of −10° relative to its flat state, and thus receives illumination cone IC at a −10° angle of incidence relative to this “on” state. The resulting “on” state projection pupil ON is thus reflected at an angle of 10° from micromirror M in its “on” state, which is at a nominal angle of 0° from the flat state. Conversely, micromirror M is deflected to an angle of +10° relative to the flat state for an “off” pixel, resulting in an angle of incidence of illumination cone IC of −30°, an angle of reflection of 30° for the “off” state projection pupil OFF, which is at an angle of 40° relative to the flat state of micromirror M.
In practice, the flat state of micromirror M is not used in operation. As such, the “projection pupil” FLAT illustrated in FIG. 1 contains light corresponding to noise, such as light reflecting from micromirror M during transitions between the “on” and “off” state, reflection from flat surfaces of the digital micromirror device (e.g., the package window, border metal, etc.). This separation distance between the projection pupils ON and OFF is beneficial, of course, to reduce stray interference of the light of “off” pixels from reducing the contrast of the image projected by the “on” pixels.
The angles of deflection of micromirror M define various attributes of the projection system. For example, as evident from FIG. 1, the angles of deflection define the maximum angle subtended by the projection pupils ON, OFF, FLAT without directly interfering with one another. For example, in modern micromirror spatial light modulators in which the micromirrors are capable of deflecting to angles of ±12°, the ON, OFF, and FLAT projection pupils can each subtend an angle of 24° without directly interfering with one another. These 24° projection pupils correspond to a numerical aperture, or f-number, of f/2.4.
Conventional projection display systems separate the “off” pixel light from the “on” pixel light prior to the “on” pixel light reaching the projection lenses. FIG. 2a illustrates a conventional micromirror-based projection display system, such as described in commonly assigned U.S. Pat. No. 6,824,275, incorporated herein by this reference. In this example, illumination “engine” 12 generates sequential-color light, by way of a conventional lamp and color wheel system, or by way of a set of primary color lasers and a scroller, as known in the art. This generated light is directed at total internal reflectance (TIR) prism 14, which includes an interior surface, between dissimilar materials, that is reflective to light that has an angle of incidence greater than or equal to a critical angle from the normal defined by the difference in refractive index values of the materials (typically a glass/air interface), as known in the art. Incident light at an angle less than this critical angle will be transmitted through the internal surface. As such, the internal surface of TIR assembly 14 reflects the light from illumination engine 12 toward digital micromirror device (DMD) 15, in illumination cone IC.
DMD 15 includes an array of individually controllable deflectable mirrors, each of which is associated with a pixel of the displayed image, and each of which is thus controlled by controller 13 to be deflected into its “on” position or its “off” position, depending on the brightness of the light of the illuminating primary color that constitutes that corresponding pixel in the displayed image. In this example, similarly as in FIG. 1, the “on” pixel position of a given mirror in DMD 15 is deflected toward illumination cone IC; for the example of FIGS. 1 and 2a, this “on” position is at an angle of −10° relative to the flat state, tilting toward illumination cone IC. The light from the “on” pixels, shown as projection pupil ON in FIG. 2a, is directed to TIR prism 14, but at an angle less than the critical angle of its internal reflective surface, and as such is transmitted through that internal surface to projection lens system 16, which focuses and directs that light to display screen 10, displaying the image.
Conversely, the “off” position of each mirror in DMD 15 in the example of FIG. 2a, similarly as shown in FIG. 1, is at an angle of +10° relative to the flat state, tilted away from illumination cone IC. This light from the “off” pixels, shown as projection pupil OFF in FIG. 2a, is also directed to TIR prism 14, also at an angle less than the critical angle of the internal reflecting surface of TIR prism 14, so this light is thus also transmitted by TIR prism 14. As a result of the deflection of the mirrors in DMD 15, projection pupil OFF is directed to absorbing light dump 18, which absorbs this light to reduce stray reflection in the display system.
The arrangement of FIG. 2a thus utilizes TIR prism 14 as an angular analyzer that filters the illumination light on its way to DMD 15 from the reflected light from DMD 15. However, TIR prism 14 does not itself substantially separate the “off” pixel light from the “on” pixel light (although refraction at an external surface of TIR prism 14 may assist in this separation, as shown in FIG. 4 of the above-incorporated U.S. Pat. No. 6,824,275). As such, the distance between the first of projection lenses 16 and the surface of DMD 14 must be sufficient to ensure that the “off” projection pupil OFF does not overlap into the projected light, as it reaches projection lenses 16. In this arrangement, this distance between DMD 15 and the first of projection lenses 16 is occupied by TIR prism 14, as shown in FIG. 2a. While a greater distance between DMD 15 and the first of projection lenses 16 will further facilitate the separation of “off” from “on” pixel light, increasing this distance will necessarily require increasing the size of the enclosure for the projection system. In addition, this distance is related to the back focal length of projection lenses 16 for a given numerical aperture, and thus an increase in the back focal length will require an increase in the diameter of the optics of projection lenses 16. As known in the optics art, this increase in lens size and in back focal distance substantially increases the cost of manufacture of the lenses, especially for relatively “fast” numerical apertures such as f/2.4 and faster. In addition, the complexity of lens design required for aberration correction also increases with increasing aperture size.
As mentioned above, and as evident from FIG. 2a, the efficiency of conventional micromirror-based display systems is also degraded by the loss of that light that illuminates “off” pixels at the DMD. However, it is known to incorporate retro-reflection recycling of the “off” pixel light back into the illumination, thus improving the efficiency of the system and the brightness of its displayed images. Such an arrangement is illustrated in FIG. 2b, in which spherical mirror 19 replaces light dump 18 from the system shown in FIG. 2a. Spherical mirror 19 is effectively concentric with projection pupil OFF, so that off” pixel light is directed back toward DMD 15, as shown by ray RCYC in FIG. 2b. Those micromirrors in the “off” state will thus reflect this reflected light RCYC back to TIR prism 14. Because the reflected light RCYC will be incident on the internal reflecting surface of TIR prism 14 at the same angle of incidence as the light from projection engine 12, this recycled “off” pixel light RCYC will reflect from this internal surface back toward the illumination source. As shown in FIG. 2b, the illumination engine includes light source 12a and integrator 12b. Integrator 12b may be a rod integrator or other similar apparatus known in the art for recovering and recycling reflected light, as described in the above-incorporated U.S. Pat. No. 6,771,325 and U.S. Pat. No. 7,118,226. Integrator 12b will thus receive the recycled “off” pixel light RCYC from TIR prism 14, and recover at least some of that energy as source light to be redirected to DMD 15 via TIR prism 14.
The recovery and recycling of “off” pixel in this known manner is useful in improving the efficiency of the micromirror-based display system, and thus the brightness of the displayed image. However, this known technique has been observed to reduce the contrast of the displayed image, because of the substantial light scattering involved in the redirecting of the “off” pixel light along its same path back to the illumination engine. Such scattering results from diffraction of the recycled light by the various elements in its return path, as well as diffraction resulting from the inefficiency of anti-reflective coatings at these high angles of incidence. In addition, DMD 15 itself causes substantial scattering from those individual micromirrors that are in the “on” position, as well as backside reflectivity and other scattering from mirrors as they make transitions between the states (especially in pulse-width-modulated systems), and also causes diffraction losses inherent to the pixelized nature of DMD 15 itself. In addition, the coupling of the recycled light RCYC from mirror 19 back through DMD 15 and TIR prism 14 is less than ideal, because the “off” pixel light must pass through these elements. For example, an efficiency of only about 62% for the recovery of “off” pixel light in an arrangement as shown in FIG. 2b has been observed, in connection with this invention. Furthermore, this arrangement tends to complicate the focal plane of the projection system, from the standpoint of projection lens system 16.
By way of further background, my copending and commonly assigned U.S. patent application Ser. No. 11/693,343, filed Mar. 29, 2007, incorporated herein by this reference, describes a micromirror-based projection television display system that can be housed in an enclosure that is competitive with modern LCD and plasma display systems. As described therein, conventional micromirror-based projection systems typically require larger “form factor” enclosures, than do LCD and plasma flat-panel systems of similar screen size and resolution, particularly in connection with the “chin” dimension and the “depth” of the enclosure. The display system described therein can be housed in such a competitively-sized enclosure, with excellent optical and thermal performance, because of the arrangement of its projection lenses, including telecentric projection lenses in a first group, followed by a medium-to-wide angle aspheric projection lens formed of plastic with >1.0 magnification, and a plastic aspheric mirror that reflects the projected image to the display screen.
Because of the compact enclosure sizes of display systems using the competing LCD and plasma display technologies, micromirror-based display systems are now subject to extremely tight constraints in their illumination and projection systems. These constraints limit the ability to separate “off” pixel light from “on” pixel light using lens elements that are not cost-prohibitive, yet providing the desired field of view. The recycling of “off” pixel light in such a constrained system, using conventional design techniques, is effectively not possible, without enduring the inefficiencies and increased scattering described above relative to FIG. 2b. 
Another constraint faced by the designers and manufacturers of modern micromirror-based projection display systems is the necessity for “fast”, or large aperture, projection lenses. An example of this constraint is illustrated in FIG. 2c, and is determined by the angle Θtip to which an individual DLP mirror M is deflectable. According to this known design constraint, an angle Θmax is defined as the maximum angle of light projection that the usable aperture of projection lenses 16 (i.e., the first projection lens 160 as shown in FIG. 2c) must be capable of receiving. This angle Θmax, measured from the normal of mirror M in its flat state, is related to the mirror deflection angle Θtip by the relationship:Θmax=3(Θtip)+f/#where f/# is one-half of the angle subtended by the reflected light pupil. For the example of angle Θtip of 12°, and a projection pupil having a numerical aperture of f/2.4 (i.e., subtending 24°), the resulting angle Θmax is 48°. Because this angle Θmax is measured from the normal, and because this angle in this example is a relatively wide angle (i.e., indicates a relatively fast lens), the numerical aperture of projection lens 160 must be at least as fast as f/0.68 in order to receive the projection pupil from this mirror M, according to conventional calculations.
A projection lens having a numerical aperture of at least as fast as f/0.68 indicates that its optics must be relatively large, especially if its focal length (distance to its rear focal plane) is of any substantial length. As such, for purposes of cost and optical quality, the distance between DMD 15 and this first projection lens 160 is preferably minimized, such as in high numerical aperture microscope objectives, which can attain more than 50× magnification with front focal distances of on the order of one or two hundred microns. However, referring back to FIGS. 2a and 2b, the presence of TIR prism 14 between DMD 15 and projection lens group 16 is a substantial cause of the lengthening of this focal length. In addition, this distance between DMD 15 and first projection lens 160 is also somewhat necessitated in order to separate the “off” and “on” pixel light projection pupils as mentioned above, to maximize contrast. And also, as mentioned above, the retro-reflection of light in the manner illustrated in FIG. 2b can improve the brightness of the displayed image, but will tend to reduce contrast because of the additional scattering involved. Furthermore, this arrangement is vulnerable to additional scattering of the retro-reflected light, and resulting loss of brightness and degraded contrast, due to inefficiency in the anti-reflective coatings of the lens elements at these high angles of incidence.
Another constraint presented to the designer of modern rear projection micromirror-based display system derives from the size of the DMD, and its position within the field of view of the projection lenses. Modern micromirror-based display systems now use a DMD of a size on the order of 0.45 inches diagonally. In rear-projection systems with constrained enclosure sizes, such as described in the above-incorporated application Ser. No. 11/693,343, the image area from a DMD of this size is required to fit within the field of view presented by the projection lens rear group, but offset from the optical axis of the front group of projection lenses, for example in the manner illustrated in FIG. 2d, in which the DMD image 15I is offset by 110% from on-axis. Optically, the overall projection aperture A must accommodate the offset projection pupil. For example, if the projection pupil for DMD image 15I has a numerical aperture of f/2.8, the numerical value of aperture A must be about f/0.68 or faster. In addition, these constraints increase the difficulty of retro-recycling the “off” pixel light in such a system.